1. Field of the Invention
The present invention relates to an exposure apparatus and a method of measuring an exposure condition, and more particularly, to a projection exposure apparatus and a method of measuring an exposure condition in an exposure apparatus. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for measuring an optimum exposure condition in the projection exposure apparatus for a short period of time.
2. Discussion of the Related Art
In an exposure apparatus used in manufacturing highly integrated devices such as semiconductor devices, the line density of a transferred pattern has been higher, while the wavelength of exposure light in the apparatus has been shorter. It is thus required to control exposure conditions, such as an exposure amount and a focal position, even more precisely. However, it is not easy to obtain an optimum exposure amount and an optimum focal position. The optimum exposure amount is generally defined by a type of light source, reticle (or mask) and photoresist, and material of wafer. If a mercury-vapor lamp or an excimer laser is used as a light source, the illuminance of exposure light applied to a mask diminishes as time passes. When a phase-shift reticle or half-tone reticle is used to improve the resolution, the amount of light reaching a wafer (for example, a photosensitive substrate) varies with the specific type of reticle. This results from the different permeability of each phase-shift reticle or half-tone reticle. An absorption ratio of exposure light also various depending upon the material or thickness of the underlayer of the photoresist applied to the wafer. Accordingly, the optimum exposure amount also depends on the absorption ratio. An antireflection resist applied on the wafer can change the optimum exposure amount. On the other hand, the optimum focal position is not readily standardized because it depends on the manufacturing processes of the mask and the photosensitive substrate.
To this end, pre-exposure is conventionally performed to obtain the optimum exposure amount and the optimum focal position. In other words, test exposure is performed prior to actual exposure. Test exposure is performed, for example, on each of a plurality of exposure areas on the wafer while changing the parameter values of the exposure conditions, such as the exposure amount and focal position. After the wafer subjected to the test exposure is developed, the exposure result is observed by naked eyes or a microscope to determine the optimum exposure condition. Since the image-forming position varies with the exposure location on the wafer due to distortion of the image plane, test exposure should be applied to every exposure area on the wafer.
Recently, a step-and-scan exposure method has been proposed to enlarge the exposure area (a shot area) on the wafer. In the step-and-scan exposure method, an illumination light having a slit-like profile is emitted to the a reticle and the reticle and the wafer are synchronously moved with respect to the illumination light to transfer a reticle pattern onto the exposure areas on the wafer. This type of exposure apparatus also requires test exposure for every shot area. Test exposure is performed, while changing the exposure parameters, to create a matrix-type shot map representing the exposure state of each area.
However, in the recent step-and-scan method, the number of parameters, such as an exposure amount and focal position, affecting the optimum exposure condition increases, and the conventional method can not collect sufficient exposure parameters in the number of exposure areas (or shot areas) formed on a photosensitive substrate is limited in certain areas. Moreover, it is difficult for the conventional method to expose a pattern under a number of different exposure conditions.
In a step-and-scan exposure apparatus, since the dimension of an exposure area on the wafer is large, the number of shots (shot area) in one wafer decreases. Accordingly, the number of exposure conditions decreases. Further, if a pattern is exposed twice at different focal positions, the number of parameters for the test exposure increases, so that more shot areas are needed on the wafer to determine optimum condition. Also, since the reticle and the wafer are simultaneously scanned and moved for the test exposure, the step-and-scan method takes more time to obtain the optimum exposure condition.
Accordingly, the present invention is directed to a method of measuring an exposure condition for an exposure apparatus that substantially obviate problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of measuring an exposure condition in the projection apparatus for a short period of time.
Additional features and advantages of the invention will be set forth on the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and the advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the exposure method of the invention is suitable to obtain an optimum exposure condition for a scanning-type projection exposure apparatus that forms an illumination area on a mask, and moves the mask and a photosensitive substrate synchronously relative to the illumination area to transfer the pattern of the mask onto the photosensitive substrate. In another aspect of the present invention, an exposure method includes the first step of moving the photosensitive substrate in the stepwise manner, while maintaining the mask at rest, to successively expose plurality of sub-areas using predetermined pattern images on the photosensitive substrate while changing an exposure condition, and the second step of measuring the exposure condition based on the state of the predetermined pattern images formed in the respective sub-areas.
Preferably, one or more sub-area, in which substantially satisfactory pattern images are formed, are selected among the plurality of sub-areas. Then, the mask and the photosensitive substrate are synchronously moved to successively expose a plurality of shot areas on the photosensitive substrate using predetermined pattern images, while changing the exposure condition in the vicinity of the level of the exposure condition obtained in the selected sub-areas. The dimension of the shot area is set larger than that of the sub-area The exposure condition include an exposure amount and a position in the direction parallel to the optical axis of the illumination light.
Prior to obtaining the optimum exposure condition according to the steps described above, the luminance may be measured at a plurality of points within the illumination area on the photosensitive substrate. Fluctuation in the luminance of the illumination area may be detected based on the measured luminance.
In another aspect of the present invention, a plurality of sub-areas on the photosensitive substrate are successively exposed using predetermined pattern images with the mask being at rest, while changing the exposure conditions. This arrangement can form more exposure sub-areas on the photosensitive substrate, as compared with the case in which the mask and the photosensitive substrate are synchronously moved during exposure. Accordingly, the number of parameters used for determining the exposure conditions can be increased. This means that test exposure can be performed under various levels of the exposure condition, and the optimum exposure condition can determined with fine resolution.
Among the plurality of sub-areas exposed as described above, one or more sub-areas having substantially satisfactory pattern images formed therein are selected. Then, the mask and the photosensitive substrate are synchronously moved to exposure each of a plurality of shot areas on the photosensitive substrate, while changing the exposure condition in the vicinity of the level of the exposure condition obtained from the selected sub-areas.
In a further aspect of the present invention, a method of obtaining an optimum exposure condition of a step-and-scan exposure apparatus, the step-and-scan exposure apparatus synchronously moving a mask and a substrate relative to an illumination light to transfer a pattern of the mask onto a plurality of shot areas on the substrate, the method comprising the steps of determining an unevenness of an illuminance of the illumination light, adjusting the illumination light to change the illuminance within a predetermined range, inputting a plurality of first exposure conditions into the step-and-scan exposure apparatus, testing the plurality of first exposure condition in a step-and-repeat mode.